1. Field of the Invention
The present invention is in the field of additive manufacturing, more specifically in the field of solid state additive manufacturing. In particular, the invention is in the field of additive friction-stir fabrication of metallic substrates for addition of stiffening ribs and/or repair of defects.
2. Description of Related Art
Friction-stir welding is a process for solid state joining of pieces of metal at a joint region through the generation of frictional heat at the joint and opposed portions of the metal pieces by cyclical movements of a tool piece that is harder than the metal pieces (e.g., non-consumable tool). An example of this is provided by International Application Publication No. PCT/GB1992/002203. Frictional heat produced between the substrate and the tool during the process causes the opposed portions to soften, and mechanical intermixing and pressure cause the two materials to join. Typically, two materials are placed side-by-side and are friction-stir welded together at the seam between the two materials. In the metalworking arts, a number of attempts have been made to fabricate complex structures including metal sheets that are joined to one or more ribs, or similar structures. Many of these efforts have used variations of friction stir welding and linear friction welding processes to join a preformed rib to another piece of metal. As used in the context of this specification, the term preformed means any substrate (e.g., a rib) manufactured to have a predetermined thickness, shape, and size. Various methods for creating complex metal structures are described in U.S. Patent Application Publication Nos. 2009/0236403A1 and 2012/0325894A1, and U.S. Pat. Nos. 6,193,137; 6,276,591; 6,290,117; 6,413,610; 6,582,832; 6,599,641; 7,240,821; and 8,061,579.
In contrast, friction-stir fabrication, invented by the present inventors (see U.S. Pat. Nos. 8,636,194; 8,632,850; 8,875,976; and 8,397,974, the contents of which are hereby incorporated by reference in their entireties), is an additive process for joining materials along a continuous plane rather than along a seam. Additive friction-stir fabrication (FSF) processes use shear-induced interfacial heating and plastic deformation to deposit metallic coatings onto metal substrates. FSF coatings have bond strengths superior to those of thermally sprayed coatings, and have the potential to enhance corrosion resistance, enhance wear resistance, repair damaged or worn surfaces, and act as an interfacial layer for bonding metal matrix composites. In this process, the coating material, such as a metal alloy, is forced through a rotating spindle to the substrate surface. Frictional heating occurs at the filler/substrate interface due to the rotational motion of the filler material, such as a rod, and the downward force applied. The mechanical shearing that occurs at the interface acts to disperse any oxides or boundary layers, resulting in a metallurgical bond between the substrate and coating. As the substrate moves relative to the tool, the coating is extruded under the rotating shoulder of the stirring tool.
Conventional additive manufacturing techniques for manufacturing complex structures include three-dimensional printing capable of adding ribs above the surface plane. However, these conventional additive manufacturing techniques are based on melting and deposition. The solid state processes for fabricating ribs do not have finer resolution, making these processes unsuitable for efficiently manufacturing very thin ribs, with or without lower through thickness properties. Thus, there is a need in the art for new additive manufacturing techniques for manufacturing complex structures.